Support thread safe unsafeWrite about bitvec

[treeowl]

For the thread-safe version, you should be able to import Data.Vector.Unboxed.Base to convert, for free, between Unboxed mutable vectors of words and Primitive mutable vectors of words. Then you can use whatever atomic operations are offered by primitive, and/or whatever else you need from GHC.Prim (as re-exported from GHC.Exts).

[Bodigrim]

My draft looks as follows:

basicUnsafeWrite (BitMVec s _ (U.MV_Word (P.MVector o _ (MutableByteArray mba)))) !i' (Bit True) = do
    let i     = s + i'
        I# j  = o + divWordSize i
        k     = modWordSize i
        I# kk = 1 `unsafeShiftL` k
    primitive $ \s ->
        let (# s', _ #) = fetchOrIntArray# mba j kk s in
            (# s', () #)

This thread-safe implementations appears to be almost twice slower than unsafe one, so I think it is worth to wrap it into its own newtype. I am not sure which of two should be provided to users as "default".

[Bodigrim]

Done.